Certain clinical and morphologic cardiac observations are described in 18 necropsy patients aged 18-58 years (mean 45) (14 women) with chronic hypercalcemia (11.6-34.4 mg./d1 [mean 19.4]) for 1-9 years (mean 5). Primary hyperparathyroidism was present in 9 patients and secondary hyperparathyroidism in the other 9. Cardiac anular and extensive coronary arterial calcific deposits were present in 10 patients (group I) including 4 with considerable narrowing of 2 or 3 of the 4 major epicardial coronary arteries. Of the other 8 patients (group II), none had cardiac valve anular or cuspal calcific deposits; only 2 had coronary calcific deposits and in each they were small and did not cause significant luminal narrowing. Calcium was in the media ("medial calcinosis") in the coronary arteries in 5 patients. Comparisons of the patients in group I to those in group II disclosed similar mean ages, durations of hypercalcemia and serum calcium levels, but significantly (p less than 0.05) higher mean total serum cholesterol levels (216-vs.-163 mg./d1) and heart weights 426-vs.-320 g.). Thus, chronic hypercalcemia is associated with accelerated deposition of calcium in the cardiac anuli and valvular cusps, in the intima and media of coronary arteries and in individual myocardial fibers, and the coronary intimal calcium may produce luminal narrowing, especially in patients with serum total cholesterol levels greater than 200 mg./d1. Chronic hypercalcemia, therefore, may be viewed as a "risk factor" to accelerated coronary atherosclerosis.